The setup files for xdm are under /etc/X11/xdm. You can change various appearance issues in Xresources, and you can specify a program to start, such as a console, clock, or toy, in Xsetup. When a user logs in, the Xstartup script is run as root, and then the Xsessionscript is run as the user, usually starting a window manager and sometimes ausefulprogramortwo. This provides the backbone for a customizable user setup of a Unix system.